This invention relates to an improved process for preparing a foamed article and to a die therefor. The primary object of the present invention is to obtain a foamed article having a flat and smooth surface which is difficult to mar.
It is known that a foamed article can be obtained by extruding a foamable thermoplastic resin. It is also known that a foamed article can be prepared by a process which comprises using a die having a resin discharge plate provided with a number of apertures, extruding a foamable resin from the apertures to form a plurality of resin strands or members, and coalescing the resin strands into a foamed article. Further, it has also been known that the quantity of the individual resin strands can be controlled as desired, when use is made of the die having said apertures, by varying the dimensions of the apertures, the distances between contiguous apertures, or the land lengths of the apertures. For example, U.S. Pat. No. 3,720,572 discloses that a foamed article can be obtained having resin strands of desired densities in the desired positions thereof, by employing a die which is provided with a number of apertures appropriately varied in dimensions and distributions, foamable resin strands are extruded from the apertures and coalesced into a unitary article. The patent also discloses that the foamed article thus obtained has on the surface a pattern resembling straight grains in the natural wood.
According to the disclosure in U.S. Pat. No. 3,720,572, a foamed article is to be obtained in which the resin strands in the surface portion are foamed to low foaming degrees and the surface is hard to mar, since in general, the process disclosed therein can give a desired density to each of the resin strands, regardless of their position in the article. Additionally, especially when polystyrene is used as a raw material, it is easy to form in the surface of each of the resin strands a layer which is foamed to a low foaming degree and has a high density. In practice, however, it is not always possible to achieve a surface layer foamed to the desired low foaming degree and high density, and the article is liable to be marred when used in certain fields of application. Furthermore, articles produced according to the known process may occasionally rupture along the coalescing surfaces between the strands, owing to imperfect coalescence of the strands.
The present invention aims to improve such defects. The inventors attempted to develop a process for preparing a foamed article, the surface of which is hard to mar and which has coalescing lines which appear as if they are straight grains in natural wood, by extruding a foamable resin into a number of resin strands and by coalescing the resin strands into a unitary article immediately after they have been extruded.
The inventors tried initially to cool some resin strands alone which were positioned in the outer-most layer in the foamed article, in order to allow the resin strands to expand to lower foaming degrees. In general, cooling has heretofore been practiced when foamable resin strands are extruded and coalesced, but the cooling has only been carried out by either blasting the air from a position far away from the resin, in order to avoid direct contact of the resin strands with the cooling means, or by allowing the resin strands to contact the cooling means for only a short distance, for example, point contact at a line extending in the direction perpendicular to the direction of extrusion. Contact over a short distance was used because direct contact of the resin strands with the cooling means tends to prevent mutual coalescence of the resin strands. The inventors, however, tried to cool the foamable resin strands by allowing the members to contact the cooling means directly over a longer distance. For this purpose, the inventors mounted a cooling frame on a die, and advanced the resin strands in contact with the cooling frame over a distance of more than several mm immediately after the resin strands had been extruded from the die. Only the resin strands which were positioned in the outer-most layer were cooled by this means. As a result, it was found that the resin strands in the surface portion of the foamed article were foamed to comparatively low foaming degrees. Coalescing lines between the resin strands, however, were formed in considerably sunken states on the surface of the article, and thus the article did not have a flat, smooth and beautiful surface.
The inventors made further studies, therefore, to a develop a process which would eliminate these surface defects and, as a result, have found that it is effective to form a step-up on the resin discharge face of the die, such that an inner portion of the face projects from an outer portion of the face when viewed from the front of the resin discharge plate, and, accordingly, to extrude the foamable resin from the apertures in the outer portion at an earlier stage than the resin from the apertures in the inner portion. The inventions have also found that, in order to cause the outer resin strands along to be of high densities in this case, it is desirable to increase the diameters of the outer apertures relative to those of the inner apertures, or to distribute the apertures more densely in the outer portion than in the inner portion. The present invention is based on the above findings.